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United Nations Mission in Sudan (UNAMIS) personnel guard South Sudanese people displaced by recent fighting in Jabel, on the outskirts of capital Juba on Monday. Clashes between rival groups of soldiers in Juba a week ago have spread across South Sudan, which won its independence from Sudan in 2011 after decades of war.

The violence broke out earlier this week when a military commander defected, sparking a rebellion in Bor, a town north of the capital of Juba. Experts, however, said tensions had already been running high between ethnic Dinkas and Nuers, due to a decision by President Salva Kiir, a Dinka, to fire Vice President Riek Machar over the summer. Mr. Machar, a Nuer, has become the rallying point of an anti-government rebellion.

With growing reports of reprisal and ethnically motivated attacks, the violence threatens to escalate into full-scale civil war. Witnesses on the ground said that government security forces had executed dozens of ethnic Nuers in a region north of Juba, BBC reports.

An estimated 100,000 people have been forced to flee parts of the country as rebels have seized major towns, including some areas in the critical oil-producing region in the north. A spokesman for the South Sudanese military, Phillip Aguer, told CNN that Bentiu, the capital of the oil state Unity, was under rebel control.

Revenues from oil exports are crucial to impoverished South Sudan, as the country struggles to build a coherent, functioning state two years after declaring independence from Sudan.

UN Secretary-General Ban Ki-moon appealed to the Security Council today to reassign around 5,500 troops from other UN missions in Africa to South Sudan to help protect civilians, the BBC reports. He also asked for attack helicopters, three transport helicopters, and one military transport plane. The Security Council will meet this afternoon to vote on the resolution.

"Those responsible at the senior level will be held personally accountable and face the consequences, even if they claim they had no knowledge of the attacks," Mr. Ban said.

The US, meanwhile, moved around 150 Marines from Spain to a base in the Red Sea nation of Djibouti to prepare for the evacuation of more US citizens and other foreign nationals. A similar mission over the weekend was aborted when US aircraft came under fire from the ground, wounding four military personnel. The US State Department has called on US citizens to leave the country.

"The United States and the United Nations, which has the lead for securing Bor airport in South Sudan, took steps to ensure fighting factions were aware these flights were a humanitarian mission," State Department spokeswoman Jen Psaki said in a statement.

"The US government is doing everything possible to ensure the safety and security of United States citizens in South Sudan. We are working with our allies around the world to connect with and evacuate US citizens as quickly and safely as possible," she said.

Experts say that despite billions of dollars in international aid and efforts by organizations from around the world, South Sudan risks turning into a failed state.

According to Reuters, the two men — President Kiir and Machar — long had a problematic relationship, with Machar making no secret of his ambitions to become president.

In the year before Machar’s dismissal, the two men's relationship in office was defined by "miscommunication or mistrust or silence,” a former government official, Jok Madut Jok, told Reuters.

Control of the oil fields is also vital to Sudan, which lost the fields when the south became independent, but relies on fees from oil going through its pipeline to the Red Sea. The two countries nearly came to war last year over disputes about borders, transit fees, and other issues.